The first aim of this investigation is to define in precise terms the function of E. coli DNA ligase, E. coli DNA polymerase I, T4 DNA ligase and T4 DNA polymerase in DNA replication. A second aim is to arrive at a detailed understanding of the structure and mechanism of the E. coli DNA ligase. A third aim is the identification and characterization of as yet unknown enzymes and factors involved in DNA replication. The investigation will be organized along the following lines: 1. Further investigation of the structure, mechanism and function of DNA ligases: a. Proteolytic dissection of E. coli DNA ligase; b. Studies of Ligase-DNA interaction; c. Studies of suppression of T4 polynucleotide ligase mutations by mutations in the rII gene. 2. Studies of DNA replication in E. coli: a. Role of DNA polymerase I in DNA replication and DNA repair; (1) Purification of polymerase I from pol A12 (temperature-sensitive) and pol A5 (non-suppressible mutants); (2) Purification of pol A1 amber fragment and investigation of its enzymatic activities; (3) Introduction of the pol A12 (temperature- sensitive) mutation into pol A1 (amber) mutants; b. Studies of DNA replication in a DNA polymerase I-DNA ligase double mutant; (1) Investigation of the basis for shut-off of DNA synthesis in a polymerase I-ligase double mutant; (2) Analysis of 5'-terminal nucleotide sequences of "Okasaki fragments" generated in a polymerase I-ligase double mutant; c. Examination of E. coli mutants with abnormally high recombination rate ("hyper rec mutants") for enzymes and factors involved in DNA replication; 3. Studies of the mechanism of activation of T4 DNA polymerase.